A Grenville Sm–Nd age for the Glenelg eclogite in north-west Scotland

Abstract

Eclogite, a garnet–pyroxene rock with basaltic chemistry, is a product of unusually high-pressure metamorphism. In Scotland, it occurs within the Caledonian orogenic belt close to the Moine thrust, where it forms partially amphibolitized, easterly-dipping layers, associated with metasediments in the eastern Glenelg Lewisian inlier^1–4. In spite of its potential importance for an understanding of the geological history of north-west Scotland, its age has never been established more precisely than ‘probably Proterozoic’⁴. A K–Ar omphacite age of 1,515 Myr⁵ is rendered unreliable by the possible presence of excess argon. We report here the results of an attempt to date the eclogite metamorphism by the Sm–Nd method. Two samples of eclogite yielded mineral isochrons of just over 1,000 Myr. The results represent clear evidence of Grenville metamorphism in Scotland, and imply close proximity of its front to the Moine thrust near Glenelg. Also, as the local Moine cover has not apparently reached eclogite facies, it was presumably deposited on an eroded Grenville basement.